This invention relates to a method for the purification or recovery of orthophosphoric acid rich streams. Orthophosphoric acid in water is the common form for most solutions labeled phosphoric acid. The following descriptions may interchangeably use either term. Phosphoric Acid purified in accordance with the method according to the invention is suitable, in particular, for use in the semiconductor industry and in the flat panel display industry (LCD screens) and in other electronics industries, e.g. as etchant. In those applications, the presence of minute amounts of metal ions can already significantly impact the quality of the produced chips and circuit boards.
Ultra pure phosphoric acid is required for various other uses, as well. Its unique ability to resist oxidation, reduction and evaporation makes it particularly useful for high purity industrial and manufacturing processes. As already mentioned above, an important application is the electronics industry, where the phosphoric acid is used to etch away such parts of a photosensitive surface of a wafer that previously have been exposed to ultraviolet light.
During the etching process a significant amount of impurities will be dissolved into the acid etchant solution. The spent etchant from such applications typically contains besides phosphoric acid also nitric acid, acetic acid and the periodic table of various metallic and other impurities including, but not limited to, Na, Mg, Al, Si, K, Ca, Cr, Mn, Fe, Ni, Cu, Zn, Ag, As, Sr, Mo, Cd, Sn, Sb, Au, Pb and Bi. These are all typical of the elements that can be present in the impure acid. Due to the desired reaction between phosphoric acid in this application as an etchant and the metallic parts of the semiconductor product a significant amount of the acid has to be discharged from such process and replaced by fresh acid in order to avoid accumulation of these impurities towards an unacceptable level. Since the tolerable levels are remarkably low it is therefore economically and ecologically attractive to recover such spent acid.
Many different purification processes are known from prior art. The aim is increase the purity level of the orthophosphoric acid beyond those values, which are acceptable for other applications outside the electronics industry. Such known purification processes for higher phosphoric acid grades for use in industrial, food or pharmaceutical applications include solvent extraction, chemical precipitation, absorption, ion exchange methods among others. These known processes have the disadvantage of being complex and are typically limited to a specific impurity or type of impurity meaning the multiple processes must be executed in series to remove all the impurities. They also generate separate waste streams that must be treated separately and as a result are prohibitively expensive.
U.S. Pat. No. 3,991,165 describes an extraction process that preferentially removes the phosphoric acid while leaving other impurities in the raffinate. The final product is not suitable for electronic grade application and the process is relatively complex and requires multiple solvents and complex operation.
Chemical precipitation is another purification method found in literature. These processes are generally limited to a single impurity, require additional reactants and generate waste streams that must be further treated making them complex and expensive to implement.
Absorption methods are also described in literature but these suffer the same restrictions as chemical precipitation.
K. J. Kim and S. Y. Kim have proposed a hybrid process combining distillation and a discontinuous layer crystallization process for the purification of spent phosphoric acid etchants in Purification of phosphoric acid from waste acid mixtures, Kwang-Joo Kim, Su-Yeon Kim, Proceeding from the ISIC 2006. Such discontinuous static and dynamic layer crystallization processes are known from the prior art for other chemical applications: e.g. acrylic acid, DMT, para- and meta-Xylene among others. Such layer melt crystallization processes are characterized by relatively high crystal growth rates between 10−5 m/s and 10−6 m/s and result in an impure crystal product. The crystal lattice typically would still remain pure, but the surface grows in a dendrite like structure and mother liquor containing all the impurities gets entrapped into the multifaceted structure. It is known that such dynamic impurity inclusion effects become more pronounced with increasing viscosity. For example, the above authors describe that 4 layer melt crystallization steps are required to gradually increase the purity of a crude phosphoric acid from 77, 93% by weight to 89, 73% by weight. The separation of a single layer melt crystallization process can be enhanced by additional purification methods like sweating and washing: these methods offer an increase in purification efficiency at the expense of the product yield.
Suspension-based crystallization has been proposed as purification process as evidenced by EP0209920B1 included herein as reference.
Suspension based crystallization provides a system for continuous operation which allows for a stable seed supply as required in EP0209919B1. The large crystal mass provides a massive growth surface and allows for very slow and near ideal growth rates. The growth surface in suspension based crystallization typically exceeds 5,000 m2 of crystal growth surface per m3 of system volume and can be as high as 20,000 m2 of crystal growth surface per m3 of system volume. This far exceeds the growth surface available in layer type system which is typically limited to <100 m2 of crystal growth surface per m3 of system volume. The massive growth surface in suspension based crystallization allows the production of extremely pure crystals even in the presence of impurities that may become incorporated in the crystal lattice at much faster growth rates. The main problem with suspension crystallization in this application is the high viscosities reported in literature which should hinder the separation of the individual crystals from the impure liquid.
With such high viscosities these processes generally do not achieve the required purification and require, like the layer system, multiple steps to obtain ultra-high purities. Washing steps are included but this uses valuable product and reduces the overall efficiency of the purification step without being able to finally achieve an ultra-pure product. For example, PCT application WO 00/59827 is targeting to achieve a food grade product, only. With a content of the individual metals in the final pure product acid in the ppm range no electronic grade product could be obtained.